Re: Stego/Ankylo limbs

["Nicholas R. Longrich" <longrich@phoenix.Princeton.EDU>]

On Tue, 30 Jan 1996 Dinogeorge@aol.com wrote:

> Since ceratopians were secondarily quadrupedal (the ancestral ceratopians
> were small bipedal marginocephalians and psittacosaurs), there is no reason
> to expect their graviportal forelimbs to have had a pose as fully erect as
> their hind limbs, which had been primitively erect (a dinosaurian apomorphy)
> for about 150 million years. Indeed, because the large, graviportal
> ceratopians had evolved quite rapidly during essentially the second half of
> the Late Cretaceous (about 20 million years altogether), one would not expect
> much in the way of a fully erect pose--the result of lengthy evolution--from
> the formerly grasping forelimbs. The forelimbs became suborned in a "fast and
> dirty" way to holding up the animals' forequarters, letting the "evolutionary
> chips" fall where they may. Perhaps, had there been no terminal Cretaceous
> extinction event, the forelimbs of ceratopians would eventually have trended
> toward a more fully erect pose, but now we'll never know.

        Were those limbs really just for grasping? Psittacosaurs have 
decently powerful front arms, that seem at least as useful for walking on 
the ground as a kangaroos, so they could have had some practice. And what 
about ornithopods? Even ornithopods only partially quadrupedal seem 
to have erect forelimbs.
 Iguanodon had bipedal ancestors (dryosaurs? or somebody else?) 
but doesn't seem to have had a problem with walking on 
erect front limbs, at least part time, and, neither does Tenontosaurus, 
which is a modified hypsilophodont. 
        -nick L.